In recent years, active researches on digital modulation/demodulation have been made in the fields of satellite communications, mobile communications and mobile satellite communications. Especially in an environment for the mobile communications, a signal is generally received in a state where it has been subjected to heavy fading, and differential detection is more frequently used as compared to coherent detection for executing demodulation with stability even under the fading environment as described above. However, in the differential detection, there is such a defect that the performance is rapidly degraded if a received signal has a frequency deviation, so that the frequency deviation is necessary to be corrected, thus an automatic frequency control circuit being required.
Description is made for a conventional type of automatic frequency control circuit with reference to the related drawings.
FIG. 14 shows an example of configuration of the automatic frequency control circuit based on the conventional technology in a system to which a four-phase PSK modulated signal is used.
In the figure, designated at the reference numeral 10 is a phase rotating section, at 11a, 11b an LPF, at 12 a differential detecting section comprising an angle converting section 120 and a phase differential detecting section 121, at 13 a modulated component removing section, at 14 an averaging filter section, at 15 an integrator, and at 16 a coordinate transforming section.
Next description is made for operations. At first, received signals are down-converted and are subjected to quasi-coherent detection by a local oscillator with phases displaced by .pi./2 to be converted to Ich and Qch data each as a baseband signal. Baseband signals for the detected Ich and Qch data are inputted to the phase rotating section 10 and is subjected to phase rotation according to a signal for correcting a frequency deviation outputted from the coordinate transforming section 16. The baseband signal outputted from the phase rotating section 10 is filtered by the LPF sections 11a and 11b to be inputted in the differential detecting section 12, where the signal is converted to a phase data by the angle converting section 120, and then a phase differential detection between the present phase data and the phase data delayed by one symbol is executed by the phase differential detecting section 121, namely a phase difference therebetween is detected.
From the phase difference detected by the phase differential detecting section 121, modulated components of the baseband signal are removed by the modulated component removing section 13 and a phase rate rotated according to the frequency deviation in one symbol is detected. Signals outputted from the modulated component removing section 13 are averaged by the averaging filter section 14 for removing noise components or the like therefrom. The signal outputted from the averaging filter section 14 as described above indicates a phase rate obtained by being rotated according to the frequency deviation in one symbol, and assuming that a one-symbol delay rate of a received signal by differential detection is Ts, a phase difference after removal of the modulated components detected therefrom at the time is .increment..theta. and a frequency deviation is .increment.f, the relation therebetween is expressed by the following equation, and a frequency deviation can be detected from the detected phase rate through this expression. EQU .increment..theta.=2.pi..increment.f T s
Signals outputted from the averaging filter section 14 are integrated by the integrator 15 for generating a signal for phase rotation. The signal obtained through integration by the integrator 15 is converted to cross signals by the coordinate transforming section 16 and inputted to the phase rotating section 10. Namely in the phase rotating section 10, a frequency deviation in a received signal can be corrected by rotating a phase against the received signal so that the detected frequency deviation will be canceled out.
By the way, in a case of the four-phase PSK modulation which is a modulating system often used in the satellite communications, mobile communications and mobile satellite communications or the like, a signal is required to be multiplied by 4 to remove modulated components from the received signal for detecting a frequency deviation therein.
For this reason, an area in which a frequency deviation can be detected from the received signal is restricted to within .+-.1/8 of a symbol rate in a case where one-symbol delay is employed, so that it is impossible to sweep a received signal in a normal phase control point in the automatic frequency control circuit based on the conventional technology having the configuration as described above in a case where the frequency deviation exceeding .+-.1/8 of the symbol rate is generated in the received signal.
Also in a case where a frequency deviation due to the Doppler fluctuation arises after the received signal is once captured, the automatic frequency control circuit will track the frequency deviation, but the frequency tracking area will be restricted according to the maximum value of the signal outputted from the averaging filter section. For this reason, in a case where the frequency deviation due to the Doppler fluctuation or the like exceeds the tracking area, the automatic frequency control circuit can not track the frequency deviation in the received signal.